1. Field of the Invention
The present invention relates to an exposure mask, a pattern formation method, and an exposure mask fabrication method.
2. Description of the Related Art
In currently available exposure processes for semiconductor devices, an oblique incidence illumination method is employed to ensure a sufficient focal depth for dense patterns such as line-and-space patterns. Stable formation is thus ensured for patterns with considerably fine dimensions as long as they are cyclic patterns such as DRAM gate patterns or wiring patterns. The oblique incidence illumination method is a method in which a normal incidence component is cut from mask illumination light so that a mask is illuminated with oblique incident light. An ordinary illumination method forms a three-beam interference image by focusing three beams including zero-order diffracted light and positive and negative first-order diffracted light from a mask pattern by means of a projection lens. In contrast, the oblique-incidence illumination method forms a two-beam interference image by cutting off one of positive and negative first-order diffracted light components to obtain an image with two beams consisting of zero-order light and the other one of the positive and negative first-order diffracted light components.
Comparing the three-beam interference image formation with the two-beam interference image formation in terms of best focus, the two-beam interference image formation is inferior in terms of contrast since one of positive and negative first-order diffracted light components is cut off. However, when taking into consideration an incidence angle on a semiconductor substrate serving as an image formation plane, the incidence angle in the two-beam interference image formation is a half that in the three-beam interference image formation. Therefore, in the two-beam interference image formation, the degree of image blurring due to defocusing is reduced by that much, and it is made possible to obtain sufficient light intensity distribution for formation of a resist pattern in a wide focal range. Restrictions on the direction and angle of light illuminating a mask are provided by a secondary light source formed by a fly's eye lens having a metallic aperture following an exit of a light source. This is because the original light source, which is typically a mercury lamp or an excimer laser device, cannot illuminate the mask with uniform intensity by itself. To solve this problem, the mask is illuminated with the fly's eye lens which forms an aggregate of several hundreds of point light sources.
When viewing the optical illumination system from the mask side, only the aggregate of the spot light sources formed by the fly's eye lens can be seen and hence exposure characteristics are determined by the shape of the aggregate of the spot light sources. Accordingly, the original light source such as a mercury lamp is referred to as the primary light source, and the aggregate of spot light sources formed by the fly's eye lens is referred to as the secondary light source (effective light source). Additionally, a condenser lens is arranged between the secondary light source and the mask, whereby light from a central portion of the secondary light source is made incident vertically to the mask, while light from the outer peripheral portion of the secondary light source is made incident obliquely to the mask. Therefore, the oblique incidence illumination method employs an aperture which shields light from the central portion of the secondary light source or opens allowing only the light from the outer peripheral portion. The exposure characteristics on a wafer vary depending on this aperture shape (secondary light source shape), and secondary light sources having shapes as shown in FIGS. 1A to 1C have been proposed.
An illumination shown in FIG. 1A is referred to as a two-point illumination. A secondary light source 20a having a shape as shown in FIG. 1A provides an effect of improving the focal depth of a one-direction pattern (herein, a horizontal-direction pattern).
An illumination shown in FIG. 1B is referred to as a four-point illumination. A secondary light source 20b having a shape as shown in FIG. 1B provides an effect of improving the focal depth of a two-direction (vertical and horizontal) pattern. Using the four-point illumination, the exposure characteristics of the vertical and horizontal pattern will be the same in the vertical and horizontal directions as long as the light source is located in the direction of 45/135 degrees. If the light source is deviated from this direction, the exposure characteristics will be different between the vertical and horizontal directions. For example, if the pitch in the vertical direction is relatively dense while the pitch in the horizontal direction is relatively sparse, in the example shown in FIG. 1B, the secondary light source assumes such a shape that the outermost periphery of the secondary light source is open in the vertical direction, while the inside of the outermost periphery is open in the horizontal direction.
An illumination shown in FIG. 1C is referred to as an orbicular zone illumination. A secondary light source 20c having a shape as shown in FIG. 1C provides high versatility since the exposure characteristics have no dependency on pattern directions. Therefore, the orbicular zone illumination is employed in general, and if sufficient focal depth cannot be obtained with the orbicular zone illumination, then possibility of employment of the four-point illumination or two-point illumination applicable to limited patterns is studied.
There have also been proposed illumination methods for improving the exposure characteristics by utilizing polarization instead of the shape of the secondary light source. For example, Japanese Laid-Open Patent Publication No. H07-183201 (Patent Document 1) discloses a method for improving the exposure characteristics of a pattern in a specific direction by polarizing exposure light so as to be TE polarized light in the specific direction pattern even when the orbicular zone illumination is employed.
It is known that the focal depth can be enlarged further by the use of a halftone phase shift mask. The term “focal depth” refers to a focal range in which an effective resist pattern can be obtained. The term “halftone phase shift mask” refers to a phase shift mask that is obtained by forming a mask pattern functioning as a shielding region to be a translucent region so as to allow about 2 to 20% light to leak through, and inverting the phase by 180 degrees between the leaked light and light from a peripheral transparent region. If the pattern is a line-and-space pattern generating diffracted light, the use of a halftone mask together with the oblique incidence illumination method, the balance between the zero-order diffracted light and positive first-order (or negative first-order) diffracted light is improved, resulting in improvement in contrast.
As for an isolated pattern generating no diffracted light, however, the deformation illumination method described above does not have significant effect and the focal depth is not enlarged so much. The focal depth of the isolated pattern can be enlarged more effectively by reduction of numerical aperture (NA) or reduction of coherence factor (σ). The coherence factor σ refers to a ratio of size of an illumination lens to a size of a pupil plane of a light source. This means that σ is equal to a quotient obtained by dividing NA of an illumination lens by NA of a projection lens. The coherence factor σ is one when the size of the illumination lens is the same as the size of the pupil plane of the projection lens. Reduction of NA in an optical illumination system means that a mask is illuminated only with a substantially vertical light component. The focal depth can be improved more by illumination with low σ also when a halftone phase shift mask is used. All these conditions for enlarging the focal depth of an isolated pattern may lead to deterioration of resolution of a dense pattern. This makes it difficult to achieve favorable exposure characteristics both for a dense fine patterns and an isolated pattern.
In order to solve this problem and to provide a method capable of achieving improved focal depth both for a dense pattern and an isolated pattern, there has been studied a method of using a so-called auxiliary pattern, a fine pattern not resolved in itself. Such an auxiliary pattern is described for example in Japanese Laid-Open Patent Publication No. H04-268714 (Patent Document 2). Patent Document 2 employs a method of improving the focal depth of a pattern, according to which when a mask is illuminated with oblique incident light, an auxiliary pattern having a dimension smaller than a critical resolution is arranged in the vicinity of the pattern while aligning the angle and direction of the auxiliary pattern with those of the oblique incident light. By using the mask having the auxiliary pattern arranged as described above under the oblique-incidence illumination condition, the image formation condition approaches the two-beam interference image formation and thus the focal depth is enlarged.
The position and dimensions of the arranged auxiliary pattern affect the focal depth of a device pattern. While an optimal value for an interval between the auxiliary pattern and the main pattern differs depending on their dimensions and optical conditions used, the optimal value falls within a range of about 1.5 times of a critical resolution of the optical conditions. Although the effect of enlarging the focal depth of the main pattern becomes higher as the dimension of the auxiliary pattern is increased, the auxiliary pattern itself will be transferred onto a wafer if the dimension thereof is too great. Therefore, the size of the auxiliary pattern is set slightly smaller to allow some margin than the limit of size not causing the auxiliary pattern to be transferred onto the wafer.
A rule-based method and a model-based method have been proposed as a method of arranging an auxiliary pattern. The rule-based method is a method in which a table for designing a mask pattern is preliminarily prepared according to intervals between a main pattern and a pattern next to the same. According to the rule-based method, an auxiliary pattern is arranged for each of all the main patterns according to a rule, and then any problems in the auxiliary patterns such as insufficient spacing between the auxiliary pattern and the main pattern or insufficient spacing between the auxiliary patterns. The rule-based method has an advantage that the auxiliary pattern can be generated and checked rapidly. In addition, since the arrangement rule is preliminarily prepared, the result can be checked easily by using design rule check (DRC).
The model-based method is a method in which simulation is performed to find a value of contrast or focal depth, and an auxiliary pattern is generated if the value is insufficient. The model-based method has an advantage that the auxiliary pattern can be arranged to ensure sufficient exposure characteristics.
On the other hand, there have also been proposed halftone phase shift masks having a shielding region in addition to a transparent region and a translucent region. Such masks are also referred to as tritone masks, and those of rim type having a translucent region only in the vicinity of a hole pattern are well known. Japanese Laid-Open Patent Publication No. 2000-19710 (Patent Document 3) discloses a tritone mask having a translucent region of an opposite phase in the vicinity of a hole pattern, an octagonal translucent region of the same phase formed around the translucent region, and a shielding region formed around the outer periphery of the translucent region. Using the tritone mask, the focal depth can be enlarged since the amplitude distribution of mask transmitted light is allowed to have pseudo periodicity by using the translucent region and shielding region having the same phase.